Suppose a user signs in with Google. Then later the user for some reason gets locked out of their Google account. Can they also sign in with Facebook and access the same Cognito account (Assuming they have the same email address registered with both Facebook and Google)?
Yes you can add multiple federated identities to a single User Pool. Just add multiple providers in cognito user pool settings. And yes the account is the same so if you lose your google account you will still have access through facebook as long as your email address for both of these services is the same.